1. Field of the Invention
The present invention relates to a system for detecting faults in the insulation of conductors in motors and, more particularly, to a system for detecting insulation faults bysensing the ground leakage current flowing in the ground wire cable of the motor drive circuit.
2. Description of the Related Art
High dv/dt switching of PWM inverter motor drive systems results in multiple paths of common mode leakage current in the motor drive system. A generalized circuit diagram of a motor drive system is provided in FIG. 1. As shown in FIG. 1, the common mode leakage paths exist in the:
1. Capacitance coupling 2 between the grounded heatsink and the high voltage switching devices (i.e., IGBTs, etc.);
2. Capacitance coupling 4 between the motor lead cable and the grounded conduit or a cable tray;
3. Capacitance coupling 6 between stator winding of a motor and the motor frame through insulation material.
Leakage paths 2 and 4 above do not vary over time in the operation of an AC motor drive inverter system. Thus, for capacitance coupling path 2, the rate of change of voltage at switching instances (dv/dt) does not vary as a function of time or aging so long as the gate drive impedance stays the same. Therefore, the resulting common mode leak current does not change over time. Likewise, with respect to capacitance coupling path 4, since motor lead cabling does not change over time, the common mode leak current associated with this capacitance coupling path does not change.
Capacitance coupling 6, between the motor stator winding and the frame through insulation material, will, however, vary over time as a result of insulation deterioration within the motor. It is important to detect this insulation degradation in order to prevent catastrophic motor insulation breakdown.
The present invention provides a circuit for detecting motor insulation degradation by monitoring the common mode leakage current flowing in the ground wire cable of the motor controller circuit. More specifically, as the motor insulation degrades, the dielectric characteristics of the insulation also change. This insulation degradation manifests itself at the ground cable from the motor frame as an increase of common mode leakage current. At the end of the motor life, the common mode leakage current is so significant that it can be detected by the motor drive control system.
The present invention thus provides a circuit for detecting motor insulation degradation by detecting the common mode leakage current in the ground wire cable of the motor. In a first preferred embodiment, the circuit of the present invention includes a current sensor (for example, a small current transformer) for sensing current flowing in the ground wire cable), a single pole low pass filter for generating an average leakage current signal from the sensed leakage current, a pulse width modulator circuit connected to the current transformer and the low pass filter for generating a pulse width modulated signal representative of the common mode leakage current, a pulse generator receiving the pulse width modulated signal and generating a pulse signals at the rising and falling edges of the pulse modulated signal, a pair of level shifters for transposing the voltage level of the pulse signals from a floating high voltage level to a voltage level referenced to ground, and a pulse reconstruction circuit for receiving the level shifted pulse signals and reconstructing the pulse width modulated signal at the voltage level referenced to ground which is representative of the magnitude of the common mode leakage current and can be inputted directly to a microcontroller or a digital signal processor to determine the amount of motor insulation degradation.
In a second embodiment of the present invention, a second winding on a toroidal core, in addition to a winding connected to an active EMI filter amplifier, is used to sense the common mode leakage current signal flowing in the ground wire cable. This embodiment of the invention advantageously generates a common mode leakage signal referenced to the low side ground, and accordingly, the sensed signal need not be transposed and is simply conditioned and fed to a peak detector circuit, the output of which may be compared to a threshold value to generate an insulation fault trip signal.
In a further embodiment of the present invention, the common mode leakage current is sensed by a transformer core located on a DC bus link, rather than on the ground bus. In this configuration, the normal mode current is automatically canceled and only the common mode current is sensed.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.